1. Field of the Invention
The invention relates to an exhaust gas purifying system for an internal combustion engine, which adds a reducing agent to a portion upstream of an exhaust gas purifying catalyst that is provided in an exhaust passage of the internal combustion engine, and an exhaust gas purifying method.
2. Description of the Related Art
In a known purification technology, a storage and reduction type NOx catalyst (hereinafter referred to as “NOx catalyst”) is provided in an exhaust passage of an internal combustion engine, in particular, in the exhaust passage of a diesel engine, so as to remove nitrogen oxides (hereinafter, “NOx”).
When the NOx catalyst is provided in the exhaust passage, it is necessary to remove NOx or sulfur oxides (hereinafter, “SOx”) stored in the NOx catalyst by discharging and reducing such substance before a NOx storage capacity of the NOx catalyst is saturated. In order to do so, an air fuel ratio of exhaust gas flowing into the NOx catalyst needs to be made rich (equal to or smaller than a stoichiometric air fuel ratio).
However, in the diesel engine, the amounts of hydrocarbon (hereinafter, “HC”) and carbon monoxide (hereinafter, “CO”) contained in exhaust gas are relatively small, and a chance that the air fuel ratio of exhaust gas becomes equal to or smaller than the stoichiometric air fuel ratio is very rare. Therefore, it is difficult to obtain the reducing agent, which is used for reducing oxides stored in the NOx catalyst, from exhaust gas discharged from cylinders. In order to solve this, a reducing agent injection valve is provided upstream of the NOx catalyst in the exhaust passage. The reducing agent is injected from the reducing injection valve, so that the air fuel ratio of exhaust gas flowing into the NOx catalyst becomes equal to or smaller than the stoichiometric air fuel ratio on an as-needed basis.
As described above, it has been suggested that the reducing agent is injected from the reducing agent injection valve at a timing synchronized with the exhaust stroke of the cylinders of the internal combustion engine (as an example, refer to Japanese Patent Application Publication No. JP-A-6-117224).
Various methods other than synchronization with the exhaust stroke of the cylinder of the internal combustion engine have been considered and studied so as to add the reducing agent more preferably.